polycystins
Polycystins belongs to the VIC Superfamily. The observations that polaris (TTC10), polycystin-1 (PKD1) and polycystin-2 (PKD2) are expressed in the primary cilia of polarized renal epithelial cells, and that inversin (INVS) and nephrocystins (NPHP1, NPHP2, NPHP3, NPHP4) are expressed in cilia, led to the following axiom: the products of almost all genes that are mutated in CKDs in humans, mice or zebrafish are expressed in primary cilia, basal bodies or centrosomes.
The unexpected convergence of mechanisms that underlie CKDs pathogenesis is evident from the finding that clinical phenotypes are related to tissue-specific expression in the sensory cilia.
Further evidence comes from the fact that most of these proteins interact with each other and from the high evolutionary conservation of the proteins that are involved.
Members
polycystin-1 (PKD1)
polycystin-2 (PKD2)
Pathology
polycystin-1 (PKD1) and polystin-2 (PKD2) are mutated in human autosomal dominant polycystic kidney disease (ADPKD)
polycystin-L, very similar and probably orthologous to PKD2, is deleted in mice with renal and retinal defects.
Function
PKD1 and PKD2 interact to form the non-selective cation channel in vitro, but PKD2 can form channels in the absence of any other associated protein. They share a homologous region of about 400 residues (residues 206-623 in PKD2; residues 3656-4052 in PKD1) which includes five TMSs of both proteins. This may well be the channel domain.
PKD2 and polycystin-L have been shown to exhibit voltage-, pH- and divalent cation-dependent channel activity.
PKD1 may function primarily in regulation, both activating and stabilizing the polycystin-2 channel (Xu et al., 2003).
Pathogenesis
The pathogenic link between cystoprotein expression in cilia, basal bodies and centrosomes and the renal cystic phenotype remains unknown. Nevertheless the following hypotheses have been put forward on the basis of what is known about the mechanosensory function of primary cilia, the role of centrosomes in cell-cycle regulation, and the expression of cystoproteins at focal adhesions and adherens junctions.
The mechanosensory hypothesis
Based on the initial finding that bending the primary cilium elicits a Ca2+ influx, it was shown that cilia can function as mechanosensors to sense fluid movement in the kidney tubule in cooperation with polycystins.
In this model polycystin-1 (PKD1) transmits the signal to polycystin-2 (PKD2), which is a TTC calcium channel. This produces sufficient Ca2+ influx to induce Ca2+ release from intracellular stores, which then regulates numerous signalling activities inside the cell that are linked to cell-cycle regulation. It is thought that defects in cell-cycle regulation might be ultimately responsible for the development of kidney cysts.
In support of the ciliary hypothesis of CDK, motor proteins of the kinesin II family, KIF3A and KIF3B, which are involved in intraciliary transport, were shown to cause renal cystic disease in the mouse when mutated.
Kramer-Zucker et al. recently showed that cilia of larval zebrafish kidney tubules have a 9+2 configuration and are motile. Disruption of cilia structure or motility resulted in pronephric cyst formations, with left-right asymmetry defects.
Despite the amount of data that are in support of the ciliary hypothesis, some data are hard to reconcile with this model.
One of the CDK variants, medullary cystic kidney disease 2 (MCKD2), is an autosomal dominant disease that is caused by mutations in uromodulin (UMOD), the expression of which has so far not been seen in cilia, basal bodies or centrosomes. In addition, studies in the Tg737 and orpk mouse models of CDK showed that cyst formation and cilia morphology are independent phenotypic features that are regulated by polaris.
Focal adhesion hypothesis
When NPHP1 was first identified, a pathogenic hypothesis was proposed that tied in nephrocystin-1 (PKD1) with defects of cell-cell and cell-matrix signalling.
This was based on the fact that nephrocystin-1 (PKD1) contains an SH3 domain, localizes to adherens junctions and focal adhesions of renal epithelial cells, and interacts with integral components of these structures, such as BCAR1.
This ’adherens junction/focal adhesion hypothesis’ of NPHP pathogenesis has recently been partially reconciled with the ’cilia and centrosome’ hypothesis in an integrative hypothesis by showing that nephrocystin-4/nephroretinin (NPHP4) in polarized epithelial cells co-localizes with beta-catenin (CTNNB1) to cell-cell contact sites and to primary cilia, whereas in dividing cells it localizes to centrosomes.
The JAK-STAT signalling pathway
Evidence for a role of polycystin-1 (PKD1) in renal tubulogenesis came from many studies. As an example, exogenous overexpression of polycystin 1 in Madin-Darby canine kidney (MDCK) cells resulted in a reduced proliferation rate, increased resistance to apoptosis, and the development of branching tubules rather than cysts.
Polycystin-1 (PKD1) and polyctin-2 (PKD2) signalling and the renal cystic phenotype might be linked by a role of these proteins in cell growth-regulation.
Polycystin-1 (PKD1) expression activates the JAK-STAT pathway, thereby upregulating p21 and inducing cell-cycle arrest in G0 and G1. The cell-cycle arrest requires polycystin-2 (PKD2).
The involvement of polycystin-1 (PKD1) or polyctin-2 (PKD2) signalling in the JAK-STAT pathway might explain how mutations of either gene can result in dysregulated growth.
The Wnt pathway
Some results might connect bending of primary cilia by tubular flow to maintenance of normal tubular development and morphology by yet another mechanism. In this model Wnt signalling occurs primarily through beta-catenin-dependent pathways in the absence of tubular flow.
Stimulation of the primary cilium by flow is thought to result in increased expression of inversin, which then reduces levels of cytoplasmic dishevelled (DSH) (by increasing its proteasomal degradation).
Reduced levels of DSH will switch off the canonical pathway by allowing activation of the beta-catenin destruction complex.
If inversin (INVS) is defective (as in NPHP2), switching from the non-canonical to the canonical Wnt pathway might interfere with normal renal tubular morphology by disrupting apical-basolateral polarity of the renal epithelium.
Because inversin (INVS) changes its localization in a cell-cycle-dependent fashion (it can be found at the mitotic spindle, midbody, basal body or centrosomes, or cell-cell junctions) it is unclear which of these localizations is central to the pathogenesis of the renal cystic phenotype. It is also conceivable that co-localization of inversin with cadherin to cell-cell contacts might be directly related to the CDK phenotype.
Other signalling pathways of polycystins. Polycystin-1 (PKD1)activates G-protein G subunits, thereby functioning as a G protein coupled receptor.
By activating G proteins, including the heterotrimeric G proteins, CDC42 (cell-division cycle) and RAC1 (ras-related C3 botulinum toxin substrate 1), polycystin-1 promotes transcription of activator protein 1 (AP1), in a JNK-dependent and PRKCA (protein kinase C)-dependent manner.
Polycystin-1 also stabilizes cytosolic beta-catenin through modulation of the Wnt signalling pathway which has a role in the late phases of renal tubulogenesis in the developing kidney.
Cell cycle
The cell cycle is an important aspect of cystoprotein biology that might help us to understand how ciliary defects lead to CKDs at the molecular level.
Because cystoproteins change their subcellular localization according to the cell-cycle stage, it will be important to study protein-protein interactions in relation to the respective cell-cycle stages.
A balance between hyperproliferation and apoptosis might have an important role in CDK pathology. For example, PKD kidneys are grossly enlarged, whereas in NPHP and BBS kidney size remains normal and cysts grow at the expense of normal tissue.
It seems that hyperproliferation might be the predominant mechanism in PKD-like diseases, although increased apoptosis is present at the same time, whereas apoptosis might be predominant in diseases of the NPHP and BBS group.
The role of apoptosis was in fact confirmed in the Bbs2 and Bbs4 mouse models. It was recently shown that BBS4 is instrumental in recruiting proteins to the pericentrosomal matrix, implicating the centrosome and cell-cycle regulation in the pathogenesis of BBS2.
See also
cystoprotein complexes
renal tubulogenesis
sensory cilia
primary cilia
basal bodies
centrosomes
the ciliary hypothesis of CKDs
References
Pan J, Wang Q, Snell WJ. Cilium-generated signaling and cilia-related disorders. Lab Invest. 2005 Apr;85(4):452-63. PMID: #15723088#
Boletta A, Germino GG. Role of polycystins in renal tubulogenesis. Trends Cell Biol. 2003 Sep;13(9):484-92. PMID: #12946628#
Nauli SM, Zhou J. Polycystins and mechanosensation in renal and nodal cilia. Bioessays. 2004 Aug;26(8):844-56. PMID: #15273987#
Nauli SM, Alenghat FJ, Luo Y, Williams E, Vassilev P, Li X, Elia AE, Lu W, Brown EM, Quinn SJ, Ingber DE, Zhou J. Polycystins 1 and 2 mediate mechanosensation in the primary cilium of kidney cells. Nat Genet. 2003 Feb;33(2):129-37. Epub 2003 Jan 06. PMID: #12514735#